Lowering your soil pH
By Kim Willis These articles are copyrighted and may not be copied or used without the permission of the author.
There are other plants, such as rhododendrons, azaleas and
camellias that also require acidic soil.
Many gardeners have the mistaken idea that they can add some soil
amendments and quickly change alkaline soil to acidic to grow these plants but
amending soil pH isn’t that simple.
Soil is composed of minerals from dissolved and crushed
rocks, and of organic matter, air and water.
The types of rocks that formed the soil in an area added different
minerals to the soil. Hydrogen gets into
soil from those rocks, from the breakdown of organic matter and from chemical
reactions in plant root systems. In
soil, the hydrogen ions are in the water that fills the pores between soil
particles. The more Hydrogen ions in the
soil the more alkaline it is.
How acidic or alkaline the water in soil is determines what
kinds of mineral elements get dissolved and become available to plants or get
bound up in complex reactions and become unavailable. The term pH stands for the potential or
percentage of Hydrogen ions in a solution.
(The correct way to write this term is lower case p, upper case H.) A pH scale is a way to rate how acidic or
alkaline soil is. It ranges from
0-14. Seven is considered neutral. Above
7 is alkaline and below 7 is acidic.
Soil pH increases or decreases by ten times for each point
on the pH scale. A pH of 6 is ten times
more acidic than 7 and a pH of 5 is ten times that or one hundred times more
acidic than a pH of seven. So, if you
are trying to lower a soil pH of 7 to a pH of 5 for acidic soil loving plants
you are trying to make it 100 times less alkaline. You may guess from this that lowering soil pH
might not be easy and your guess would be right.
Most plants grow best at pH levels of 6.5 to 7.5. That is
where most beneficial minerals become available to them. The pH level in soil also affects the
microbes and micro-organisms that break down organic matter that adds nutrients
to soil.
A few plants have evolved to survive in soil pH levels
slightly higher or lower than that 6.5 -7 range. Plants that prefer acidic soil
like blueberries, function best when minerals like iron are easily obtained
from soil. Iron is dissolved in the acidic water of low pH soils and is
essential to plant health. Above a 6.5
pH iron is not easily available to plants as it is bound to other soil
compounds.
The plants that grow in neutral or alkaline soil evolved
chemical reactions in the root system and associations with microorganisms that
help them obtain iron bound in the soil. Blueberries and other acidic soil
plants don’t have the adaptations that other plants have evolved to take that
iron (and a few other minerals) from the soil.
Some new research has found that planting turf grasses
around blueberries and keeping them mowed may help blueberries exist in more
alkaline soil. That’s because the grass
plants have the ability to take iron from the soil and the blueberries can
“steal” some of that iron from the grass roots.
The grass roots also encourage the soil microorganisms that can release
iron in the soil to colonize.
Lowering soil pH to
plant blueberries – and other acidic soil lovers
Depending on the type of soil, what you are going to use to
lower the Ph and how much you need to lower the pH it can take a year to 3
years to substantially lower soil ph.
It’s hard to adjust soil pH when plants are already in the soil, so
don’t plant your blueberries until you have achieved the right soil pH.
If you want to plant blueberries in alkaline soil you needed
to start preparing that soil two years before you plant. You cannot just add
peat and a sprinkle of acidic fertilizer to the planting hole and hope to get
good results, even though you’ll see this advice offered by many who don’t know
anything about soil science.
The first step is to get a soil test to see what the actual
soil pH is. You can get your soil tested
at almost any County Extension office in the United States. Many garden and farm service stores also
offer the service. They will tell you
how they want you to collect and submit the specimen. These places will generally give you
recommendations when you get the results for fertilizing or changing the soil
pH.
There are small kits that have you mix water and soil and
test the pH but these are not very reliable.
If you are a person who likes to do it yourself, you can purchase a
small meter that has probes that go into moist soil and reads the pH. The more expensive ones used by professional
growers are pretty accurate.
While compost, manure, pine bark and peat may lower the pH a
tiny bit, generally after years of applying healthy quantities of these
products to the soil, you really need something like sulfur, aluminum sulfate
or ferrous sulfate to substantially lower soil pH.
Some researchers believe aluminum sulfate may harm
blueberries if it needs to be used in large quantities. So, sulfur, which is
cheaper and easier to obtain than ferrous sulfate, is probably the best
acidifier for home gardeners. The amount
to use is tricky and depends on a number of factors. The soil texture will affect the amount needed,
with clay soils needing more acidifier than sandy ones. The starting Ph is also a big factor.
Here’s a quick, rough reference for the amount of sulfur to
use per 10 sg. feet for a soil with loam texture to lower pH to 5, which is
good for blueberries.
Current pH 8 use 6/10 pound
Current pH 7.5 use 5/10 lb
Current pH 7 use 4/10 lb
Current pH 6.5 use 3/10 lb
Current pH 6 use 2/10 lb
If you use ferrous sulfate increase the amount times 6. If
your soil is really sandy reduce the amount of sulfur or ferrous sulfate by 1/3
and if its heavy clay increase it by 1/2.
You add the sulfur in spring or summer and work it into the soil. Try to
work it in to the top 12 inches of soil. You cannot add more than 2 pounds of
sulfur or 5 pounds of ferrous sulfate per 100 square feet in one
application. You will need to divide the
amount needed and add applications 3 months apart. Some acidifier products will have directions
on the package to follow.
Using sphagnum peat
to lower pH
While adding sphagnum peat to a planting hole won’t work
well, if you have money and time it can be used to lower pH over a larger area.
You need sphagnum peat, not the ground peat sold cheaply in bags in garden
centers. This method is expensive and
lots of work.
Spread the peat 3 inches deep over the soil surface and work
it into the top 8-12 inches of native soil. The area you add sphagnum peat to
should be at least 5 square surface feet per blueberry plant. Let it rest a
month and soil test for pH. You may need
to add more peat or an acidifier product.
Let it rest and test. You may
need to repeat several more times. Soil
pH does not change quickly.
When the pH is correct then dig planting holes. The
gardener’s golden rule of thumb for planting anything is to refill the hole
with the soil taken out of it, and not to add things to the hole. When the peat has been thoroughly mixed into
the area the plants will be planted in, it is unlikely to cause the problems it
would cause if you simply added it to planting holes.
If you add things like peat to the hole, which holds water,
you will create a bathtub of water in clay soil to rot plant roots. In sandy soil the peat will hold water and
the plant roots will stay in the area with the peat, until all the water is gone. The roots won’t be able to support the top
growth well then and the plant will become stunted or may die. You want plant roots to spread out into the
native soil immediately after planting, going deep and far to find water and support
the plant.
Container and raised
bed planting
If you have alkaline soil it may be easier to plant
blueberries in deep raised beds or in containers. It is cheaper to make an acidic planting
medium in these and you can often plant the same year. Raised beds and containers should be at least
12 inches deep, 18 would be better, and they must drain well. Compact and dwarf
blueberries need 2-3 square feet of surface area, normal sixed blueberries need
about 5 square feet of surface area.
A mixture of equal amounts of compost, sphagnum peat, sand
and shredded bark can be used. Add some acidic fertilizer per label directions
for containers. Thoroughly mix the ingredients and test the pH level. Add more
peat or an acidifier if it’s still too high.
When the level is good you can plant.
You’ll need to test the pH each year because rain and irrigation water
may change the pH and use either acidifiers to lower pH or if it’s below 4.5
use lime to raise it a bit.
Mulching blueberry plants is always good, but mulch won’t
contribute much to lowering soil pH, no matter what you use. Anything just added to the surface will not
lower soil pH very much.
After reading this you may decide blueberries are too much
work and expense unless you have naturally acidic soil. There’s nothing wrong and a lot to commend,
if you decide to buy your blueberries from someone who actually has the right
soil for growing them.
There are many fruits that will grow in neutral or slightly
alkaline soil such as raspberries and blackberries. You may want to try honeyberries which have a
similar tasting fruit to blueberries, and which ripen about the same time as
strawberries ripen. They are not as
fussy about soil pH.
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